Maik,
One other issue is that some of the line work appears to be contour lines. These are the light and dark blue lines to the left side. There are a few other light blue lines throughout the file that do not appear to be contour lines. Contour lines should be imported as feature type contour to improve triangulation. I noticed this when I went back to look at the generated triangles and I notice that around the blue line work on the left, I had triangle verticies that were NOT coincident with feature points. There error is very small but it is there.
Paul